The magneto-rheological fluid has the properties that the fluid viscosity changes from low to high levels along with the change of the applied magnetic field. By taking advantage of the above-mentioned properties, the viscosity of the magneto-rheological fluid can be freely changed to absorb the impact according to the strength of the impact force. The magneto-rheological fluid can absorb any impact generated in the three phases, i.e., a gas phase, liquid phase and solid phase. In the absence of magnetic field, the magneto-rheological fluid shows low viscosity and behaves like a flexible spring. When the magnetic field is applied, the magnetic force gradually increases the viscosity of the fluid as if the spring tends to be rigid.
The magneto-rheological fluid plays a role of something like spring or damper because of the action of absorbing impact, as mentioned above. In the apparatus required to have the impact-absorbing effect, the magneto-rheological fluid can contribute to absorption of the impact by changing the strength of the magnetic field applied to the fluid so as to adjust the viscosity of the fluid. In the apparatus performing rhythmic piston movements, the viscosity of the fluid can be changed according to the conditions, which makes it possible to modify the piston movement in a desired manner, i.e., intermit the movement for a short period or long period of time, and repeat the cycle of operation.
U.S. Pat. No. 6,547,986 discloses a magneto-rheological grease composition comprising magnetic-responsive particles, a carrier fluid and a thickening agent, where polyurea is shown as an example of the thickening agent. JP 2006-253239 A discloses a urea-based grease, in particular comprising an alkyl-substituted diphenyl ether and an urea compound; and a magneto-rheological fluid containing a dispersion medium and magnetic particles. However, the dispersion stability of the magnetic particles and the magneto-rheological properties are still insufficient, so that there is an increasing demand for further improvement.